Tension Leg Platform With Improved Hydrodynamic Performance

ABSTRACT

A tension leg platform that is stable with a quayside-integrated deck without the use of temporary stability modules or specialized installation techniques. The hull preferably consists of four radially-oriented vertical corner columns connected with four central ring pontoon segments. The vertical columns are fixed to the outer periphery of the central pontoon. The columns are characterized by a major radial axis and a minor transverse axis. The mooring system includes tendons supported at tendon porches directly at the four column outboard lower corners, without additional radially-extending tendon support structures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to tension leg platforms, such as foroffshore oil and gas drilling and production, and more particularly to atension leg platform that has ample inherent stability so as to allowfor quayside integration of the superstructure, towing of the integratedhull and topsides to the installation site, and installation, allwithout the use of temporary stability modules or other specializedequipment.

2. Background Art

In the offshore oil and gas industry, floating vessels such as tensionleg platforms (TLPs) for drilling and/or production are common. A TLP isa type of floating platform that is used for drilling and production inrelatively deep water. A typical TLP hull configuration consists of one,three, or four vertical columns, and three or four pontoons, whichconnect the columns below the water surface. The columns and pontoonsare typically rectangular or cylindrical in cross section. Carried ontop of the columns is the superstructure, which includes one or moredecks that support the topsides production facilities, drilling system,production risers, and living quarters, etc. At its installed draft, theTLP's pontoons are submerged and the columns extend from below to abovethe water's surface.

The mooring system of a TLP includes tubular steel members calledtendons (also referred to as tethers) which are highly tensioned becausethey are connected to a buoyant, submerged or partially submergedplatform hull. High tendon stiffness reduces the system's verticalnatural periods to a level well below that of the dominant wave energy.As a result, dynamic amplification of vertical motion is nearlynon-existent and the platform has limited heave, roll and pitch motions.The highly tensioned tendon system also limits horizontal offsets to avery small percent of water depth.

FIG. 1 is a top view (in a horizontal cross-section taken through thecolumns) of the hull of a conventional TLP 200 of prior art. Fourcolumns 212 are arranged to form a square pattern, with the axialcenterline VC of each column 212 forming one corner of the square. Fourindividual pontoons 214 form each side of the square. Pontoons 214 aretypically positioned so that their axial centerlines HC are alignedbetween column centerlines VC. Tendon porches 220 are mounted directlyto the outboard corners of columns 212 for connecting the mooringtendons.

FIG. 2 is a top view (in a horizontal cross-section taken through thecolumns) of a newer generation TLP 300 of prior art, known as anextended tension leg platform (ETLP). Like the prior art TLP 200 of FIG.1, in the ETLP 300 of FIG. 2, the corner columns 312 are located suchthat the vertical centerlines VC of the columns 312 intersect the axialcenterlines HC of the pontoons 314 connected thereto. As compared to aconventional TLP 200 of FIG. 1, which has a mooring footprint of similardimensions, the ETLP of FIG. 2 differs by positioning the columns 312and pontoons 314 more inboard to form a smaller square. Four tendonsupport structures 330 are mounted to the outboard corners of columns312 at keel level. Tendon porches 320 are mounted to the distal ends oftendon support structures 330 for connecting the mooring tendons.

Because the columns 312 are located closer to the platform center C, asimplified deck structure may be used resulting in greater structuralweight efficiency than the TLP 200 of FIG. 1. The smaller ring-shapedpontoon structure 314 also contributes to a greater structural weightefficiency and simplifies construction, reduces support spans andcantilevers, and provides improved hydrodynamic performance of theplatform. In other words, a greater payload can be supported for a givencombined weight of the hull and superstructure. Furthermore, the ETLP300 of FIG. 2, with its simplified superstructure, may allow for moreeconomical topsides integration at quayside, or eliminate the need forheavy lift vessels or float-over deck installation.

For both the TLPs 200 and ETLPs 300, the interior of both the columnsand the pontoons are typically compartmentalized by structural bulkheadsfor damage control, to strengthen the structure, to provide enclosedspaces for locating and storing various equipment (e.g., anchors,chains, propulsion mechanisms, etc.), for storage of liquids such asfuel water, and hydrocarbon products, and for ballasting.

Depending on its configuration, the stability of a TLP (conventional orextended) may be inadequate during installation. When a TLP is ballastedbetween the initial free floating draft (e.g. the wet-tow draft orfloat-off draft) and the lock-off draft (the draft at which securing theTLP to the tendons is initiated), there is a range of drafts at whichthe TLP stability is critical—the TLP may be unstable or only marginallystable prior to being locked-off to the tendons and de-ballasted.

There are a number of ways to address this stability problem whentransiting the installation drafts before lock-off and de-ballasting.Most prior art installation techniques rely on using costly specializedinstallation equipment. For example, one option is to install thetopsides deck offshore, after the hull is connected to the tendons.Offshore installation of the deck is an expensive and high riskoperation, because it typically requires the use of heavy-lift vesselsor float-over deck installation techniques. Moreover, it requires arelatively long window of good weather. Accordingly, it is generallypreferable to integrate the superstructure quayside and tow thecompleted platform to the installation site, if possible.

Another method employs the use of an upward hook load to the integratedTLP by a larger installation-support vessel. A hook load has theadvantage of being able to quickly tension the tendons after lock-offwithout waiting for the slow de-ballasting process. However, only a verylimited number of vessels exist worldwide which are capable of providingthe required hook load for a TLP of ordinary size.

Yet another method to increase stability during platform installation isto use temporary buoyancy modules to keep the hull from capsizing beforeit can be secured to its mooring tendons and subsequently de-ballasted.For example, U.S. Pat. No. 6,503,023, issued to Huang, et al. on Jan. 7,2003, discloses an ETLP that employs temporary stability modules locatedoutboard of columns above the tendon support structures. The Huang etal. method permits the TLP structure, including platform, deck andequipment to be constructed in an upright position, towed to aninstallation site, and installed by ballasting the structure ortemporary stability modules. Because the Huang et al. arrangementincreases the structure surface area at the waterline, which subjectsthe ETLP to greater wave action in the wave zone at the sea surface,after the ETLP is locked off and de-ballasted, the temporary stabilitymodules are preferably removed.

U.S. Pat. No. 5,551,802, issued to Wybro on Sep. 3, 1996, and U.S. Pat.No. 7,044,685, issued to Wybro et al. on May 16, 2006, disclose methodsfor installing a TLP in which hold-down or pull-down lines (or chains)are used at each corner of the TLP to prevent the TLP from capsizingprior to tendon lock-off. The hold-down or pull-down lines are fastenedat their lower ends to the upper tips of the installed tendons. Thelines pass through the tendon porches and then through ratchetinggripper members or winches located above the tendon porches. As the TLPdraft increases for receiving the tendons into the tendon porches, thegrippers or winches maintain tension in the lines, thus preventing theTLP from toppling to any one side.

As an alternative to these specialized installation techniques, TLPs canbe designed to have inherent stability necessary for tow andinstallation. A combination of wider column spacing and/or largercolumns or a design change that raises the metacentric height of theplatform, such as lowering the center of mass, may be used to increasestability. For example, the conventional TLP configuration of FIG. 1inherently has greater stability than the ETLP configuration of FIG. 2,all else being equal. As the design of any complex system requirestrade-offs between competing requirements, the conventional TLP designof FIG. 1 gains greater stability than the ETLP design of FIG. 2 at theexpense of sacrificing structural weight efficiency and hydrodynamicperformance.

U.S. Patent Publication No. 2002/0090270 in the name of Malcolm et al.discloses a column-stabilized semi-submersible offshore platform. TheMalcolm et al. platform employs a triangular ring-shaped pontoonstructure that is located inboard of the three corner columns.Specifically, the longitudinal centerlines of the three pontoon memberslie to the inside of the side of the triangle defined by locating thecorners at the column vertical centerlines. However, as the pontoons areonly located slightly inboard of the columns, the geometric trianglesides still pass through the pontoons but just to the outside of thepontoon centerlines.

U.S. Pat. No. 7,140,317, issued to Wybro et al., also discloses asemi-submersible platform with improved stability. The Wybro '317platform employs four columns and a rectangular ring-shaped pontoonstructure that is located inboard of the columns. That is, the sides ofthe square, defined by locating the four corners of the square at thevertical centerlines of the four columns, are located completely outsideand outboard of the pontoons. Because the Wybro '317 pontoons arelocated inboard of the columns, the platform is characterized bysimplified construction with reduced support spans and cantilevers andby improved hydrodynamic performance than if each pontoon was centeredbetween its two end columns.

Each of the semi-submersible platforms described by Malcolm '270 andWybro '317 is moored with a plurality of catenary mooring lines thatextend radially about the outer periphery of the platform. For thisreason, these platforms are not heave restrained, as is a TLP. It isdesirable, therefore, to have a heave-restrained tension leg platformthat employs a broader column spacing for enhanced stability, yet havinga smaller pontoon structure that is located inboard of the columns forimproved structural efficiency and hydrodynamic performance.

3. Identification of Objects of the Invention

A primary object of the invention is to provide a tension leg platformfor use in offshore applications, such as for offshore oil and gasdrilling and production, having a hull with a plurality of columns and acentral pontoon structure that is disposed inboard of the columns, whichsimplifies construction, reduces support spans and cantilevers, andprovides improved hydrodynamic performance of the platform.

It is another object of the invention to provide a tension leg platformhaving a hull with radially oriented rectangular columns and a centralpontoon structure disposed inboard of the columns which are formedsubstantially of flat plate construction, thus simplifying theconstruction of the structure.

Another object of the invention is to provide a tension leg platformhaving vertical columns of rectangular cross section that have majoraxis oriented radially outward from the center of the hull, whichprovide support for the deck and reduces the support spans andcantilevers of the deck structure required for deck support inconventional TLPs.

Another object of the invention is to provide a tension leg platformhaving a unitized central pontoon structure located inboard of thevertical columns that may have a central moonpool opening or may becompletely enclosed, which improves the hydrodynamic performance of theplatform as compared to conventional ring pontoon, is simplerconstruction, lighter in weight, and facilitates the support of steelcatenary and flexible risers.

Another object of the invention is to provide a tension leg platformhaving a hull with radially oriented rectangular columns and a centralpontoon structure with a moon pool, with the pontoon structure beingdisposed inboard of the columns, which allows the support of flexiblerisers on the inboard or the outboard side of the central pontoonstructure.

SUMMARY OF THE INVENTION

The objects described above and other advantages and features of theinvention are incorporated in a tension leg platform for use in offshoreapplications, such as for offshore oil and gas drilling and production,which has a hull configuration including vertical support columns, acentral pontoon structure disposed inboard of the columns at a lower endthereof, and a deck structure supported at an upper end of the columns.The structure is anchored by vertical tension legs, connected at keellevel to the outboard faces of the columns and extending verticallydownward to the seabed. The vertical mooring tendons are connected bytendon porches, which are located directly on the columns without theuse of extended tendon support structures.

The vertical columns and pontoon structure are preferably constructedsubstantially of flat plate. The vertical columns are adjoined to theouter periphery of the central pontoon and have a transverse crosssectional shape with a major axis oriented radially outward from acenter point of the hull, and a central vertical axis disposed adistance outward from the pontoon outer periphery.

Risers can be supported on the inboard or outboard side of the pontoonand extended to the deck. The central pontoon and outboard columnstructure simplifies construction, reduces support spans andcantilevers, and provides improved hydrodynamic performance of theplatform.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail hereinafter on the basis of theembodiments represented in the accompanying figures, in which:

FIG. 1 is a plan view in cross section of a conventional TLP hull ofprior art, showing pontoons disposed between and connecting verticalcolumns;

FIG. 2 is a plan view in cross section of an extended TLP (ETLP) hull ofprior art, showing vertical columns having a closer lateral spacingtherebetween (as compared to a conventional TLP hull of FIG. 1 havingthe same mooring footprint), pontoons disposed between and connectingthe vertical columns, and tendon support structures extending radiallyoutward from the columns;

FIG. 3 is a perspective view of the tension leg platform according to apreferred embodiment of the invention, showing vertical columns that areconnected together by a ring-shaped pontoon, which is located inboard ofthe columns;

FIG. 4 is a plan view in cross section taken along lines 4-4 of FIG. 3of the hull (columns and pontoons) of the tension leg platform of FIG.3;

FIG. 5 is a perspective view of the hull (columns and pontoons) of atension leg platform according to an alternative embodiment of theinvention, wherein the central pontoon structure does not have a centralopening and is located a greater distance inboard of the columns andadjoined to the columns by rectangular extensions; and

FIG. 6 is a perspective view of the hull (columns and pontoons) of atension leg platform according to another alternative embodiment of theinvention, which is similar to the embodiment of FIG. 5 except that thevertical columns have a generally trapezoidal transverse cross sectionwith a wider inboard side wall and a narrower outboard side wall.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

U.S. Pat. No. 7,140,317, issued to Wybro et al. on Nov. 28, 2006 andentitled “Central Pontoon Semisubmersible Floating Platform,” isincorporated herein by reference in its entirety.

FIGS. 3 and 4 show a tension leg platform 10 according to a preferredembodiment of the invention for use in offshore applications, such asfor offshore oil and gas drilling and production. The platform 10 has ahull 11 including vertical support columns 12 and a central pontoonstructure 14 disposed inboard of the columns at a lower end thereof. TLP10 includes a deck structure 13 supported by the upper ends of thecolumns 12.

The interior of both the columns 12 and the pontoon structure 14 ispreferably subdivided by structural bulkheads (not illustrated) tostrengthen the structure, to provide enclosed spaces for locating andstoring various equipment (e.g., anchors, chains, propulsion mechanisms,etc.), and to provide a plurality of separate tanks for purposes ofballasting the vessel and storing various fluids and other materialswhich may be required or desired during drilling or production by thewell.

TLP 10 is anchored by a plurality of vertical or near vertical mooringtendons 17 that are connected to tendon porches 18 on the lower end ofthe outboard face of the columns 12. Each column 12 is designed to matewith at least one, but usually two or more tendons 17. The tendonporches a positioned near the keel elevation and contain connectionsleeves (not illustrated) to receive the upper tips of the tendons 17and clamp thereto. The connection sleeves may be ring-shaped, requiringvertical entry of the tendons 17, or they may be slotted to allow sideentry of the tendons 17.

Various types of risers 19 can be supported by the hull 11, includingnear-vertical top tensioned risers (TTR), flexible risers, or steelcatenary risers (SCR). The flexible risers or steel catenary risers(SCRs) can be supported on the inboard or the outboard side of thecentral pontoon structure 14, and extended to the deck 13 by either asingle span spool piece or by piping supported on the hull. The toptensioned risers (TTRs) can be supported on the deck 13, and can also besupported laterally at the pontoon elevation by riser keel joints (notillustrated).

Although any suitable shape may be used, the central pontoon structure14 is preferably octagonal-shaped, having four orthogonally-orientedside segments 14 a intervaled with four diagonally-oriented cornersegments 14 b that are connected to the pontoon structure 14 to form aunitized structure centered about the platform central vertical axis C.In the embodiment shown in FIGS. 3 and 4, the central pontoon structure14 includes a central moonpool opening 14 c, which is illustrated as anoctagonal opening but may have any other suitable shape. Side and cornersegments 14 a, 14 b are each preferably characterized by generallyrectangular transverse cross section surrounding a central horizontalaxis or horizontal centerline HC.

Each of the vertical columns 12 has a lower end 12 a and an upper end 12b. The columns 12 preferably have a quadrilateral transverse(horizontal) cross-section, which may be a generally rectangular ortrapezoidal-shaped configuration. FIGS. 3 and 4 show columns 12 asrectangular, having a transverse cross-sectional shape with a major axisA₁ oriented radially outward from a center point C of the hull 11.Specifically, columns 12 define a rectangular transverse cross sectionformed of two parallel spaced wider lateral side walls 12 c connected tonarrower inner and outer side walls, 12 d, 12 e, respectively. Thus,each vertical support column 12 defines a major axis A₁ extendingbetween the inboard and outboard side walls, 12 d, 12 e, and a minoraxis A₂ extending between the two lateral side walls 12 c. Each verticalsupport column 12 defines a vertical longitudinal axis or verticalcenterline VC at the intersection of major axis A₁ and minor axis A₂.The major axis A₁ of each of the vertical support columns 12 ispreferably oriented radially outward from the center C of the platform.A lower portion of inboard side wall 12 d of each vertical supportcolumn 12 abuts and is joined to a respective diagonal corner segment 14b of the pontoon structure 14.

Vertical support columns 12 are disposed substantially outboard of thecentral pontoon structure 14. The vertical axis VC of each column 12 isdisposed a distance D₁ outwardly from the outer periphery of cornersegment 14 b of the pontoon structure 14 and a distance D₂ outwardlyfrom the central horizontal axis or horizontal centerline HC extendingthrough the pontoon corner segment 14 b. Thus, with the hullconfiguration of the present invention, the central pontoon structure 14is positioned inboard of the vertical support columns 12, such that aline S defined between the vertical centerlines VC of two adjacentcolumns 12 lies outside the horizontal centerline HC of the pontoon sidesegments and, more preferably, outside the outer periphery of thepontoon structure 14. This design feature differs from the prior arttension leg platform designs (such as illustrated in FIGS. 1 and 2),which typically have individual pontoons centered between the columns,with the vertical centerlines of the support columns intersecting thehorizontal centerlines of the adjacent pontoons.

FIG. 5 illustrates the hull 11 a of a TLP according to an alternativeembodiment of the invention. As with TLP hull 11 of the embodiment ofFIGS. 3 and 4, hull 11 a has a central pontoon structure 114 locatedinboard of the columns 12, but unlike TLP 10 of FIGS. 3 and 4, thepontoon structure 114 of FIG. 5 excludes a central moonpool opening.Additionally, the outer periphery of the pontoon structure is spaced agreater distance radially inward from the vertical support columns 12,(i.e., the pontoon 114 outer periphery is closer to the platformcenterline C). In this embodiment, the lower portion of the inboard sidewall 12 d of each vertical support column 12 is mounted and fixed to thediagonal corner portions 114 b of the pontoon structure 114 by arectangular extension 15 secured between the pontoon corner portions andinboard side wall 12 d of the column 12 to form a unitized structure.

FIG. 6 illustrates a hull 11 b of a TLP according to a third embodimentof the invention. In this alternative embodiment, each of the verticalsupport columns 112 has a lower end 112 a and an upper end 112 b anddefines a generally trapezoidal transverse cross section with a widerinboard side wall 112 d and a narrower outboard side wall 112 einterconnected in parallel spaced relation by two nonparallel laterallyspaced side walls 112 c.

According to the various embodiments of the invention, including thoseof FIGS. 3-6 and variations thereof widening the column spacingincreases stability, and placing the central pontoon structure 14, 114radially inboard of the vertical support columns 12, 112 improves thehydrodynamic performance of the platform and reduces support spans andcantilevers. And because the columns 12, 112 and pontoon 14, 114 arepreferably not cylindrical, they may be substantially constructed offlat metal plate (with the possible exception of corners, which may beprovided with either simple radius curves or sharp corners). Thisfeature simplifies the hull construction.

The Abstract of the disclosure is written solely for providing theUnited States Patent and Trademark Office and the public at large with away by which to determine quickly from a cursory reading the nature andgist of the technical disclosure, and it represents solely a preferredembodiment and is not indicative of the nature of the invention as awhole.

While some embodiments of the invention have been illustrated in detail,the invention is not limited to the embodiments shown; modifications andadaptations of the above embodiment may occur to those skilled in theart. Such modifications and adaptations are in the spirit and scope ofthe invention as set forth herein:

1. A tension leg platform (10) comprising: a hull (11, 11 a, 11 b)including a plurality of vertical columns (12, 112), a pontoon structure(14, 114) disposed inboard of said columns and adjoined to said columnsat lower ends (12 a, 112 a) thereof, each of said columns having ahorizontal cross-sectional shape that defines a major axis A₁) that isoriented radially outward from a center point (C) of said hull; a deck(13) supported at upper ends (12 b, 112 b) of said columns; and aplurality of tendons (17) connected under tension between said columnsand the seabed for maintaining said tension leg platform above a desiredsubsea location and substantially restraining said tension leg platformfrom vertical heave motions.
 2. The tension leg platform (10) accordingto claim 1 wherein: said columns (12, 112) and said pontoon structure(14, 114) are constructed substantially of flat plate.
 3. The tensionleg platform (10) according to claim 1 wherein: each of said pluralityof columns (12, 112) has a polygonal transverse cross-section.
 4. Thetension leg platform (10) according to claim 1 wherein: each of saidplurality of columns (12, 112) has a quadrilateral transversecross-section.
 5. The tension leg platform according to claim 4 wherein:each of said plurality of columns (112) has a generally trapezoidaltransverse cross-section defined by an inboard side wall (112 d) and anoutboard side wall (112 e) interconnected in parallel spaced relation bytwo nonparallel laterally spaced side walls (112 c), said outboard sidewall (112 e) being wider than said inboard side wall (112 d), wherebysaid major axis (A₁) extends between said inboard and outboard sidewalls.
 6. The tension leg platform (10) according to claim 4 wherein:each of said plurality of columns (12) has a generally rectangularhorizontal cross-section defined by an inboard side wall (12 d) and anoutboard side wall (12 e) of substantially equal width interconnected inparallel spaced relation by two parallel laterally spaced side walls (12c) of greater width than said inboard and outboard side walls (12 d, 12e), whereby said major axis (A₁) extends between said inboard andoutboard side walls.
 7. The tension leg platform (10) according to claim1 wherein: said pontoon structure (14, 114) is octagonal-shaped.
 8. Thetension leg platform (10) according to claim 7 wherein: each of saidcolumns (12, 112) is adjoined to a corner portion (14 b, 114 b of saidpontoon structure (14, 114) by an extension member (15).
 9. The tensionleg platform (10) according to claim 1 wherein: said pontoon structure(14) includes a central moonpool opening (14 c) formed verticallytherethrough.
 10. The tension leg platform (10) according to claim 9wherein: said pontoon structure (14) includes four orthogonally-orientedside segments (14 a) interconnected with four diagonally-oriented cornersegments (14 b); and each of said side segments and corner segments (14a, 14 b) are generally rectangular in transverse cross section anddefine horizontal axes (HC).
 11. The tension leg platform (10) accordingto claim 9 wherein: each of said columns (12, 112) is adjoined to one ofsaid corner segments (14 b) of said pontoon structure (14) by anextension member (15).
 12. A tension leg platform (10) comprising: ahull (11, 11 a, 11 b) including a pontoon structure (14, 114) having anouter periphery surrounding a central vertical axis (C), verticalcolumns (12, 112) each adjoined at a lower end (12 a, 112 a) to saidpontoon structure outer periphery, said pontoon structure disposedinboard of said columns, each of said columns defining a vertical columnaxis (VC) located radially outward from said central vertical axis (C) afirst non-zero distance (D₁) from said outer periphery of said pontoonstructure; a deck (13) supported by upper ends (12 b, 112 b) of saidcolumns; and tendons (17) under tension connected at upper ends to saidcolumns by tendon porches (18) mounted directly to said columns (12,112) and at lower ends to the seabed for maintaining said tension legplatform above a desired subsea location and substantially restrainingsaid tension leg platform from vertical heave motions.
 13. The tensionleg platform (10) according to claim 12 wherein: each of said columns(12, 112) has a transverse cross-sectional shape with a horizontal majoraxis (A₁) oriented radially outward from said central vertical axis (C).14. The tension leg platform (10) according to claim 13 wherein: saidpontoon structure (14) includes a vertical central opening (14 c) formedtherethrough; said pontoon structure (14) includes fourorthogonally-oriented side segments (14 a) intervaled with fourdiagonally-oriented corner segments (14 b); each of said side segmentsand corner segments (14 a, 14 b) have a polygonal vertical cross-sectionand define horizontal longitudinal axes (HC) therethrough; and eachvertical column axis (VC) of each column (12, 112) is located a secondnon-zero distance (D₂) radially outward from the horizontal longitudinalaxis (HC) of an adjacent corner segment (14 b) of said pontoon structure(14).
 15. The tension leg platform (10) according to claim 13 wherein:each of said columns (12, 112) has a polygonal transverse cross section.16. The tension leg platform (10) according to claim 15 wherein: each ofsaid columns (112) has a generally trapezoidal transverse cross sectionformed of D an inboard side wall (112 d) and a outboard side wall (112c) that is narrower than said inboard side wall, which areinterconnected in parallel spaced relation by two nonparallel laterallyspaced side walls (112 c).
 17. The tension leg platform (10) accordingto claim 15 wherein: each of said columns (12) has a generallyrectangular transverse cross section formed of an inboard side wall (12d) and an outboard side wall (12 e) of substantially equal widthinterconnected in parallel spaced relation by two parallel laterallyspaced side walls (12 c) of greater width than said inboard and outboardside walls.
 18. The tension leg platform (10) according to claim 12wherein: an imaginary line (S) extending between the vertical columnaxes (VC) of two adjacent columns (12, 112) lies outboard of saidpontoon structure outer periphery.
 19. The tension leg platform (10)according to claim 14 wherein: an imaginary line (S) extending betweenthe vertical column axes (VC) of two adjacent columns (12, 112) liesoutboard of the horizontal longitudinal axis (HC) of the side segment(14 a) located generally between said two adjacent columns.
 20. Atension leg platform (10) comprising: a ring pontoon structure (14)surrounding a central opening (14 c) and formed by a plurality ofsegments (14 a, 14 b) which in transverse cross section define ahorizontal center line (HC); vertical columns (12, 112) each adjoined ata lower end (12 a, 112 a) to an outboard side of said pontoon structure(14) so that said pontoon structure is disposed inboard of said columns,each of said columns defining a central vertical longitudinal axis (VC)disposed radially outward from a platform center (C) a distance (D₂)from said horizontal axial center line (HC) so as not to intersecttherewith; and vertical mooring tendons (17) connected between saidcolumns and the seabed.